How to exclude a category from Jekyll loop and limit the number of posts

fragment.html

{% for post in site.posts limit: 6 %}
  {% assign count = 0 %}
  {% unless post.categories contains 'talks' %}
    {% assign count = count+1 %}
    {% unless count<=3 %}
      {% include component__post-preview.html %}
    {% endunless %}
  {% endunless %}
{% endfor %}